Reasons for low iron levels?

Less in or more out. Either lack of iron intake from diet (ie. Vegetarian), lack of iron absorption (ie. Celiac), or iron losses (ie. Bleeding).
Excess loss. If you are a male or post-menopausal female it is imperative that blood loss in the stomach and intestine be excluded. Please see your doctor for that. Other cause could be low intake of iron due to poor diet.